1. Technical Field
The present invention relates to a liquid ejecting apparatus having a plurality of liquid ejecting heads arranged therein and, more specifically, to a liquid ejecting apparatus in which liquid in respective liquid ejecting heads are circulated.
2. Related Art
Examples of a liquid ejecting head which ejects liquid in a pressure chamber as liquid drops from nozzles by causing pressure variations in the liquid include an ink jet recording head (hereinafter, referred to simply as a recording head) used in an image recording apparatus such as an ink jet recording apparatus (hereinafter, referred to simply as a printer), a coloring material ejecting head used for manufacturing color filters for liquid-crystal displays, and so on, an electrode ejecting head used for forming electrodes for an organic EL (Electro Luminescence) display, an FED (Face Emitting Display), and so on, and a biological organic substance ejecting head used for manufacturing biochips (biochemical elements).
There is also a liquid ejecting apparatus including a recording head group (line-type recording head) having a plurality of recording heads arranged in the direction orthogonal to the direction of relative movement between the recording head and an object to be ejected (the direction of transporting the object to be ejected) in order to eject (discharge) liquid more efficiently and at a higher speed to the object to be ejected (recording medium). Examples of the recording head which constitutes the line-type recording head as described above include a type having a flow channel unit in which a line of a liquid flow channel from a reservoir via a pressure chamber to a nozzle is formed, or an oscillator unit having a piezoelectric oscillator which is capable of varying the capacity of the pressure chamber. There is also proposed a line-type recording head configured to cause liquid in reservoirs of the respective recording heads to circulate for the purpose of discharging foreign substance or air bubbles in the respective recording heads or for the purpose of preventing increase in viscosity of ink (for example, see JP-A-2004-167839).
Incidentally, as shown in FIG. 8A, when the liquid in the reservoir of the recording head 100 is circulated, the pressure is relatively high on the upstream side and is relatively low on the downstream side. Therefore, and hence the pressures in the pressure chambers located on the upstream side of the reservoir tend to be higher than those in the pressure chambers located on the downstream side. Therefore, the amounts of droplets to be ejected from the nozzles which communicate with the pressure chambers located on the upstream side tend to be larger than the amount of droplets to be ejected from the nozzles which communicate with the pressure chambers located on the downstream side. Then, in the line-type recording head as described above, since the directions of the flow of the liquid in the reservoirs in the respective recording heads 100 are the same, the difference in amounts of droplets to be ejected from the nozzles becomes maximum between adjacent nozzles of adjacent recording heads (see FIG. 8A). Arrows below the recording heads 100 in FIG. 8A are intended to give an idea of the amounts of droplets to be ejected from the nozzles, and show that the longer the length of the arrow, the larger the amount of droplets to be ejected from the nozzles is. In other words, in a configuration in which one hundred and eighty nozzles are provided on each of the recording head, the difference in amounts of droplets to be ejected between the nozzle No. 180 of one of the adjacent recording heads and the nozzle No. 1 of the other recording head reaches the greatest value. Therefore, for example, when ejecting the ink, the difference in amounts of droplets to be ejected is recognized as the difference in concentration of the liquid on the recording sheet (the object to be ejected) and, as shown in FIG. 8B, the difference in concentration between the recording heads are prominent as unevenness.